1. Field of the Invention
The present invention relates to plasma torches and more particularly high power plasma torches having electrodes whose life is prolonged.
2. Description of the Prior Art
Plasma torches or plasma arc blowpipes are known in the art. This type of torch consists of two electrodes, namely an anode and a cathode which are tubular and coaxial. An arc is established between the electrodes and a plasma producing gas is simultaneously injected. The arc which is struck between the electrodes is maintained and brings the gas to a very high temperature and ionizes this gas. At the outlet of one of the electrodes, this gas has high velocity and the plasma it constitutes forms the heat-carrying agent.
The arc which is struck between the two electrodes is for example initiated by contact with the aid of an auxiliary starting up electrode and then transferred between the two tubular electrodes under the action of the whirling injection of a gas in a chamber located between the electrodes. This also ensures the rotation about itself of the root of the downstream arc for avoiding the melting of the corresponding electrode. The displacement with respect to itself of the upstream arc root is obtained by an auxiliary magnetic field produced by a coil which surrounds the upstream electrode which is in the form of a glove finger with a closed end. The upstream and downstream terms are with reference to the direction of flow of the plasma.
Some types of plasma torches deliver power between 10 and 50 kWs and those to which the invention is more particularly applicable can produce several megawatts.
Such a plasma torch comprises consumable elements: the electrodes. The life of the electrodes depends on many parameters, for example the power of the torch and more particularly the value of the arc current, the nature of the plasma producing gas injected owing to its decomposition, and the reactions it may have on the materials from which the electrodes are made. The life of the electrodes is also a function of the operation of the torch, depending on whether the latter is continuous or discontinuous. It is conventional that the life of the electrodes varies between a few tens of hours for relatively low power torches to several hundreds of hours for high power torches to which the invention relates.
The relatively short life of the electrodes is a notable drawback.
In an attempt to prolong the life of the electrodes, and in particular that of the upstream electrode, there has been proposed a solution for a plasma torch whose upstream electrode is in the form of a glove finger with a closed end. According to this solution, for the purpose of acting on the wear by erosion of the electrodes, there has already been used a source of alternating current or there has already been injected plasma producing gas in the chamber between the electrodes while varying its pressure.
This apparently interesting technique is however far from completely avoiding the aforementioned drawback.
Indeed, although the life of the electrode is increased somewhat, this electrode becomes too locally worn. It would be possible to increase the worn area by increasing variations in the rate of flow of the plasma producing gas, but said variations in the flow would have an adverse effect on the constancy of the power delivered by the torch.
It has been found that, for a torch of a given power, the consumption of the mass of the electrode was an increasing function of the current and that the length of the arc was also an increasing function of the voltage. It will therefore be understood that if it is desired to increase, for a constant consumption of electrode, the power of the torch, the voltage would have to be increased, with an accompanying increase in the length of the arc, i.e. in the overall size of the installation. For practical reasons, one cannot exceed certain limits. Solutions must therefore be envisaged which permit an increase in the length of the arc while keeping as far as possible the length of the torch constant.